DTD's

XHTML documents have three parts: the DOCTYPE (which
contains the DTD declaration), the head and the body. To create web pages
that properly conform to the XHTML 1.0 standard, each page must include a
DTD declaration; either strict, transitional, or frameset. Each of the
three DTD’s is described (with an example) below:

Strict

You should use the strict DTD when your XHTML pages will be marked up
cleanly, free of presentational clutter. You use the strict DTD together
with cascading style sheets, because it doesn’t allow attributes like
"bgcolor" to be set for the <body> tag, etc.

Transitional

The transitional DTD should be used when you need to take advantage of the
presentational features that are available through HTML. You should also
use the transitional DTD when you want to support older browsers that
don’t have built-in support for cascading style sheets.

It should be fairly obvious which DTD declaration to include in your XHTML
file simply by just reading the features of each one. Now that we've got
all of the formal details of XHTML out of the way, let's look at some
examples of XHTML.